This is based on a known prosthesis (DE-A-4109941, FIG. 2) which comprises an upper contact plate for connection to an upper vertebral body, a lower contact plate for connection to a lower vertebral body, and a bridging part which connects the upper and lower contact plates and is designed for bridging at least one vertebral body which is located between the upper and lower vertebral bodies and whose function is to be replaced by the prosthesis. Between the bridging body and the contact plates, a hinge is in each case provided for replacement of the intervertebral discs. The cross-sectional size of the bridging part is considerably smaller than that of this vertebral body. If the latter is more or less completely preserved, the bridging part should be fitted into it in such a way that it is completely embedded therein. How this could be done surgically is unclear. If it is still more or less complete only on the vertebral arch side, a recess is created on its front face and the bridging part is inserted into said recess. For firm connection to the vertebral body, the bridging part has laterally protruding webs which contain an oblong hole for receiving a securing screw. The securing of the prosthesis on the vertebral body determines, in addition to the facet articulations, the position of said vertebral body with respect to the adjacent vertebral bodies. Only when those surfaces of the vertebral body intended for the securing webs to bear on are worked in such a way that the vertebral body, after connection to the webs, can maintain its natural position defined by the facet articulations, is there any prospect of the prosthesis fitting in a way that does not cause discomfort. Such precise working is difficult to achieve. It has also been found that securing by means of a screw is not reliable enough.
In another known spinal column prosthesis (U.S. Pat. No. 5,423,816), the bridging part is formed by a coil spring which, because of its compliance, has the purpose of permitting a relative movement of the upper and lower vertebral bodies with respect to one another and to the bridged vertebral body. The coil spring is intended to be received in a cavity inside the vertebral body to be bridged, which is also filled with bone chips which may possibly permit new growth of bone tissue inside the cavity. However, because of their constant relative movement, a firm connection between the turns of the coil spring and the bone tissue is not possible. Therefore, the turns of the coil spring do not form a securing means with respect to the bridged vertebral body. On the contrary, the turns of the coil spring which are being constantly moved relative to the bridged vertebral body are a cause of persistent irritation.
A spinal column prosthesis is also known (EP-A-1417940) in which the bridging part has a U-shaped configuration in side view, so as to engage with its branches on the lower face and upper face of the vertebral body to be replaced. The web lies on the front face of the vertebral body and is screwed onto it. This requires suitable working of the vertebral body on its upper, lower and front faces, which can be difficult, especially if the vertebral body is damaged. Spinal column prostheses are also known in which the bridging part completely replaces the vertebral body (EP-A-567424, WO 0103614, DE-U-20115281, U.S. Pat. No. 5,895,428). This has the disadvantage that a supporting connection is not really possible between the remaining parts of the vertebra and the bridging part.
In another known group of spinal column prostheses (U.S. Pat. No. 4,892,545, U.S. Pat. No. 4,636,217), the bridging part is connected rigidly to the upper and lower vertebral bodies, so that these too are rigidly connected to one another. The bridged vertebral body is in this way kept free from forces and therefore does not require any supporting connection to the bridging part of the prosthesis.